In the interim it has been doing shorter "monitoring" visits on new academies such as West London.

But the National Union of Teachers is suspicious. A spokeswoman said the requirement used to be an inspection within two years - why had this been changed?

"It's interesting that in the case of academies Mr Bell regards 'satisfactory' as satisfactory - in all other schools it's not good enough," she said.

"They have had millions of taxpayers' money pumped into them but they are not providing the standard of education that's required.

"In theory they replace failing schools but few are doing better and in some cases not as well."

The Commons education select committee said earlier this year that the investment in academies was welcome but the government should ensure they were thoroughly evaluated before embarking on a major expansion of "an untested model".